1. Field of the Invention
The present invention relates to a positive photoresist composition which is satisfactory in sensitivity, definition and depth of focus (DOF) properties, can form a resist pattern with a good shape especially in the formation of a mixture of an isolation pattern and a dense pattern and can minimize the formation of a back taper shape of an isolation resist pattern induced by shift of the focus to the minus side.
2. Description of the Related Art
In the manufacture of an ultralarge-scale integrated circuit (ULSI) where a high definition of not more than half a micrometer, in particular, of not more than 0.35 xcexcm is required, demands have been made on photoresist compositions which can form a resist pattern satisfactory in sensitivity, definition, and DOF properties and having a good shape.
Separately, the manufacture of a highly value-added logic integrated circuit (logic IC) has received attention in recent years. Such a logic IC has a complicated wiring pattern including large portions of irregularly formed line-and-space (hereinafter referred to as xe2x80x9cLandSxe2x80x9d). In contrast, a dynamic random access memory (DRAM) chip includes large portions of regularly formed LandS.
In a photolithographic process using a photoresist composition, demands have been made to form a resist pattern having a good pattern shape not only in regions where LandS is regularly formed, i.e., in dense pattern regions, but also in regions where LandS is irregularly formed, i.e., in isolation pattern regions.
However, in the formation of an ultrafine resist pattern of not more than half a micrometer, particularly of not more than 0.35 xcexcm, resist patterns having good shapes both in dense patterns and isolation patterns cannot be significantly formed. Specifically, if exposure conditions are changed to yield rectangular dense patterns with good shapes, the shapes of isolation patterns are deteriorated, and, in contrast, if exposure conditions are changed to yield rectangular isolation patterns with good shapes, the shapes of dense patterns are deteriorated.
Consequently, mixed resist patterns including both dense patterns and isolation patterns cannot be significantly formed with good shapes concurrently.
In addition, positive photoresist compositions are liable to invite back taper shapes of isolation patterns, which is caused by sifts of the focus to the minus side, and are liable to induce twist of pattern and exhibit deteriorated focal depth range (DOF) properties in isolation patterns.
Under these circumstances, to concurrently form a mixed resist pattern including both a dense pattern and an isolation pattern with satisfactory sensitivity, definition and DOF properties in the formation of an ultrafine resist pattern of not more than half a micrometer, particularly of not more than 0.35 xcexcm, demands have been made on positive photoresist compositions that can concurrently form a dense pattern and an isolation pattern with good shapes and can minimize the formation of a back taper shape of the isolation resist pattern induced by shift of the focus to the minus side.
Japanese Patent Application Laid-Open No. 6-167805 (Reference 1) discloses a positive photoresist composition containing, as a photosensitive ingredient, a quinonediazidosulfonyl ester of a compound represented by the following formula: 
wherein R1 is a hydrogen or halogen atom, xe2x80x94OCOR3, an alkyl or alkoxy group which may be substituted, where R3 is an alkyl or phenyl group which may be substituted; x denotes an integer of from 1 to 3; each of Q1 to Q8 independently a hydrogen atom, an alkyl group or a phenyl group; and each of Z1 to Z3 is independently a divalent phenyl group or a divalent phenyl group substituted with an alkyl group or a hydroxyl group. The reference mentions that this positive resist composition is well-balanced in definition, profile, depth of focus and other properties.
However, the reference fails to describe the formation of mixed resist patterns including both dense patterns and isolation patterns and to describe a specific structure of a compound represented by Formula (I) mentioned below for use in the present invention.
In the formation of mixed resist patterns including both dense patterns and isolation patterns, demands are made on resist compositions that can form both dense patterns and isolation patterns with good shapes and can especially minimize the formation of a back taper shape of the isolation resist patterns induced by shift of the focus to the minus side.
Japanese Patent Application Laid-Open No. 9-110762 (Reference 2) discloses a positive photoresist composition containing, as a photosensitive ingredient, a quinonediazidosulfonyl ester of a compound represented by the following formula: 
The reference mentions that this composition is a photosensitive resin composition having a high sensitivity, high definition, satisfactory profile, satisfactory focus latitude and less development scum (development residue) and is well balanced in resist properties.
However, the reference fails to describe the formation of mixed resist patterns including both dense patterns and isolation patterns. In the formation of mixed resist patterns including both dense patterns and isolation patterns, demands are made on resist compositions that can form both dense patterns and isolation patterns with good shapes and can especially minimize the formation of a back taper shape of the isolation resist patterns induced by shift of the focus to the minus side.
Japanese Patent Application Laid-Open No. 9-114093 (Reference 3) discloses a positive photoresist composition containing, as a photosensitive ingredient, a quinonediazidosulfonyl ester of a compound represented by the following formula: 
wherein each of R1 to R16 is independently a hydrogen, a hydroxyl group, an alkyl having from 1 to 6 carbon atoms or a phenyl; each of Q1 to Q10 is independently a hydrogen, an alkyl having from 1 to 6 carbon atoms or a phenyl; and each of m and n independently denotes 0 or 1. The reference mentions that this composition is a photosensitive resin composition having a high sensitivity, high definition, satisfactory profile, satisfactory focus latitude, high heat resistance and less development scum (development residue) and is well balanced in resist properties.
However, the reference fails to describe the formation of mixed resist patterns including both dense patterns and isolation patterns and to describe a specific structure of a compound represented by Formula (I) mentioned below for use in the present invention.
In the formation of mixed resist patterns including both dense patterns and isolation patterns, demands are made on resist compositions that can form both dense patterns and isolation patterns with good shapes and can minimize the formation of a back taper shape of the isolation resist patterns induced by shift of the focus to the minus side.
Accordingly, an object of the present invention is to provide a positive photoresist composition for use in the formation of an ultrafine resist pattern of not more than half a micrometer, particularly of not more than 0.35 xcexcm. This positive photoresist composition should have a satisfactory sensitivity, definition and DOF properties, can concurrently form a dense pattern and an isolation patter both with good shapes in the formation of a mixture of these resist patterns. Specifically, this positive photoresist composition could minimize the formation of a back taper shape of an isolation resist pattern induced by shift of the focus to the minus side.
After intensive investigations to achieve the above objects, the present inventors have accomplished the present invention.
Specifically, the present invention provides a positive photoresist composition including (A) an alkali-soluble resin and (B) a photosensitive ingredient, in which the photosensitive ingredient (B) includes an ester of a compound represented by following Formula (I) with a 1,2-naphthoquinonediazidosulfonyl compound: 
The aforementioned positive photoresist composition may further include (C) a sensitizer (intensifier).
In the positive photoresist composition, the ester of the compound represented by Formula (I) with the 1,2-naphthoquinonediazidosulfonyl compound preferably occupies equal to or more than 20% by weight of Ingredient (B).
In the positive photoresist composition, the ester contained in Ingredient (B) is preferably obtained by allowing 2 to 3 moles of the 1,2-naphthoquinonediazidosulfonyl compound to react with 1 mole of the compound represented by Formula (I).
Preferably, the amount of Ingredient (B) in the positive photoresist composition is from 10% to 60% by weight relative to the total amount of Ingredient (A) and Ingredient (C) which is added according to necessity.
(A) Alkali-soluble Resin
Alkali-soluble resins for use as Ingredients (A) are not specifically limited and can be optionally chosen from those generally used as film-forming substances in positive photoresist compositions. Of such resins, condensates of aromatic hydroxy compounds with aldehydes or ketones, polyhydroxystyrenes and derivatives thereof are preferable.
Such aromatic hydroxy compounds include, but are not limited to, phenol; m-cresol, p-cresol, o-cresol, and other cresols; 2,3-xylenol, 2,5-xylenol, 3,5-xylenol, 3,4-xylenol, and other xylenols; m-ethylphenol, p-ethylphenol, o-ethylphenol, 2,3,5-trimethylphenol, 2,3,5-triethylphenol, 4-tert-butylphenol, 3-tert-butylphenol, 2-tert-butylphenol, 2-tert-butyl-4-methylphenol, 2-tert-butyl-5-methylphenol, and other alkyl-substituted phenols; p-methoxyphendl, m-methoxyphenol, p-ethoxyphenol, m-ethoxyphenol, p-propoxyphenol, m-propoxyphenol, and other alkoxy-substituted phenols; o-isopropenylphenol, p-isopropenylphenol, 2-methyl-4-isopropenylphenol, 2-ethyl-4-isopropenylphenol, and other isopropenyl-substituted phenols; phenylphenol, and other aryl-substituted phenols; 4,4xe2x80x2-dihydroxybiphenyl, bisphenol A, resorcinol, hydroquinone, pyrogallol, and other polyhydroxyphenols. Each of these compounds can be used alone or in combination.
The aldehydes include, but are not limited to, formaldehyde, paraformaldehyde, trioxane, acetaldehyde, propionaldehyde, butylaldehyde, trimethylacetaldehyde, acrolein (acrylaldehyde), crotonaldehyde, cyclohexanealdehyde, furfural, furylacrolein, benzaldehyde, terephthalaldehyde, phenylacetaldehyde, xcex1-phenylpropylaldehyde, xcex2-phenylpropylaldehyde, o-hydroxybenzaldehyde, m-hydroxybenzaldehyde, p-hydroxybenzaldehyde, o-methylbenzaldehyde, m-methylbenzaldehyde, p-methylbenzaldehyde, o-chlorobenzaldehyde, m-chlorobenzaldehyde, p-chlorobenzaldehyde, and cinnamaldehyde. Each of these aldehydes can be used alone or in combination. Of these aldehydes, formaldehyde is desirable for its availability. The combination use of a hydroxybenzaldehyde and formaldehyde is specifically preferred in order to improve heat resistance.
The ketones include, for example, acetone, methyl ethyl ketone, diethyl ketone and diphenyl ketone. Each of these ketones may be used alone or in combination. Further, an appropriate combination of an aldehyde and a ketone can be employed.
The condensate of an aromatic hydroxy compound and an aldehyde or a ketone can be prepared in the presence of an acidic catalyst according to a known technique. Such acidic catalysts include, but are not limited to, hydrochloric acid, sulfuric acid, formic acid, oxalic acid, and p-toluenesulfonic acid.
The polyhydroxystyrenes and derivatives thereof include, but are not limited to, vinylphenol homopolymers and copolymers of vinylphenol with a copolymerizable comonomer. Such comonomers include, for example, acrylic acid derivatives, acrylonitrile, methacrylic acid derivatives, methacrylonitrile, styrene, a-methylstyrene, p-methylstyrene, o-methylstyrene, p-methoxystyrene, p-chlorostyrene, and other styrene derivatives.
Of these alkali-soluble resins for use in the present invention as Ingredient (A), preferred resins are alkali-soluble novolak resins each having a weight average molecular weight (Mw) of from 2000 to 20000, and typically from 3000 to 12000.
Among them, typically preferred resins are alkali-soluble novolak resins obtained by condensation reaction of m-cresol and p-cresol with formaldehyde and alkali-soluble novolak resins obtained by condensation reaction of m-cresol, p-cresol and 2,3,5-trimethylphenol with formaldehyde.
(B) Photosensitive Ingredient
Ingredient (B) is a compound having a 1,2-naphthoquinonediazidosulfonyl group. According to the present invention, an ester between a compound of following Formula (I) and a 1,2-naphthoquinonediazidosulfonyl compound is essentially used as Ingredient (B). 
The ester can be prepared, for example, by subjecting a 1,2-naphthoquinonediazidosulfonyl compound such as a 1,2-naphthoquinonediazido-4-sulfonyl halide or a 1,2-naphthoquinonediazido-5-sulfonyl halide to condensation reaction with the compound of Formula (I) to thereby yield a fully or partially esterified compound. The condensation reaction can be generally performed in an organic solvent such as dioxane, N-methylpyrrolidone or dimethylacetamide in the presence of a basic condensing agent such as triethylamine, an alkali carbonate or an alkali hydrogencarbonate.
Preferred esters are 2- to 3-mole esters which are obtained by esterifying 2 to 3 moles of the 1,2-naphthoquinonediazidosulfonyl compound with 1 mole of the compound of Formula (I), of which 2-mole esters are typically preferred.
These esters preferably occupy equal to or more than 20% by weight of, and more preferably equal to or more than 50% by weight of, Ingredient (B). If the content of the esters in Ingredient (B) is less than 20% by weight, the resulting photoresist composition may not sufficiently minimize the formation of a back taper shape of isolation resist patterns induced by shift of the focus to the minus side.
Esters of a 1,2-naphthoquinonediazidosulfonyl compound with a phenolic compound other than the compounds of Formula (I) can also be used as Ingredient (B) for use in the present invention in order to further improve sensitivity, definition and other properties. Such additional phenolic compounds include, but are not limited to, polyphenolic compounds represented by following Formula (II): 
wherein each of R1 to R8 is independently a hydrogen atom, a halogen atom, an alkyl group having from 1 to 6 carbon atoms, an alkoxyl group having from 1 to 6 carbon atoms or a cycloalkyl group; each of R9 to R11 is independently a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms; Q is a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms or Q is bonded with R9 to form a cyclic ring having 3 to 6 carbon atom members or a residue represented by following Formula (III); each of a and b independently denotes an integer from 1 to 3; d denotes an integer from 0 to 3; and n denotes an integer from 0 to 3: 
wherein each of R12 and R13 is independently a hydrogen atom, a halogen atom, an alkyl group having from 1 to 6 carbon atoms, an alkoxyl group having from 1 to 6 carbon atoms or a cycloalkyl group; and c denotes an integer from 1 to 3. The combination use of this type of polyphenolic compounds with Ingredient (A), the alkali-soluble novolak resin, can further improve high sensitivity and high definition.
Of the polyphenolic compounds, preferred compounds are 2,4-bis(3,5-dimethyl-4-hydroxybenzyl)-5-hydroxyphenol, 2,6-bis(2,5-dimethyl-4-hydroxybenzyl)-4-methylphenol, and other linear trinuclear compounds; bis[2,5-dimethyl-3-(4-hydroxy-5-methylbenzyl)-4-hydroxyphenyl]methane, bis [2,5-dimethyl-3-(4-hydroxybenzyl)-4-hydroxyphenyl]methane, bis[3-(3,5-dimethyl-4-hydroxybenzyl)-4-hydroxy-5-methylphenyl]methane, bis[3-(3,5-dimethyl-4-hydroxybenzyl)-4-hydroxy-5-ethylphenyl]methane, bis[3-(3,5-diethyl-4-hydroxybenzyl)-4-hydroxy-5-methylphenyl]methane, bis[3-(3,5-diethyl-4-hydroxybenzyl)-4-hydroxy-5-ethylphenyl]methane, bis[2-hydroxy-3-(3,5-dimethyl-4-hydroxybenzyl)-5-methylphenyl]methane, bis[2-hydroxy-3-(2-hydroxy-5-methylbenzyl)-5-methylphenyl]methane, bis[4-hydroxy-3-(2-hydroxy-5-methylbenzyl)-5-methylphenyl]methane, bis[2,5-dimethyl-3-(2-hydroxy-5-methylbenzyl)-4-hydroxyphenyl]methane, and other linear tetranuclear compounds; 2,4-bis[2-hydroxy-3-(4-hydroxybenzyl)-5-methylbenzyl]-6-cyclohexylphenol, 2,4-bis[4-hydroxy-3-(4-hydroxybenzyl)-5-methylbenzyl]-6-cyclohexylphenol, 2,6-bis[2,5-dimethyl-3-(2-hydroxy-5-methylbenzyl)-4-hydroxybenzyl]-4-methylphenol, and other linear pentanuclear compounds; and other linear polyphenolic compounds; tris(4-hydroxyphenyl)methane, bis(4-hydroxy-3,5-dimethylphenyl)-4-hydroxyphenylmethane, bis(4-hydroxy-3,5-dimethylphenyl)-3-hydroxyphenylmethane, bis(4-hydroxy-3,5-dimethylphenyl)-2-hydroxyphenylmethane, bis(4-hydroxy-2,5-dimethylphenyl)-4-hydroxyphenylmethane, bis(4-hydroxy-2,5-dimethylphenyl)-3-hydroxyphenylmethane, bis(4-hydroxy-2,5-dimethylphenyl)-2-hydroxyphenylmethane, bis(4-hydroxy-3,5-dimethylphenyl)-3,4-dihydroxyphenylmethane, bis(4-hydroxy-2,5-dimethylphenyl)-3,4-dihydroxyphenylmethane, bis (4-hydroxy-3,5-diethylphenyl)-2,4-dihydroxyphenylmethane, bis(4-hydroxy-2,5-dimethylphenyl)-2,4-dihydroxyphenylmethane, bis (4-hydroxyphenyl)-3-methoxy-4-hydroxyphenylmethane, bis(3-cyclohexyl-4-hydroxyphenyl)-3-hydroxyphenylmethane, bis(3-cyclohexyl-4-hydroxyphenyl)-2-hydroxyphenylmethane, bis (3-cyclohexyl-4-hydroxyphenyl)-4-hydroxyphenylmethane, bis (3-cyclohexyl-4-hydroxy-6-methylphenyl)-2-hydroxyphenylmethane, bis(3-cyclohexyl-4-hydroxy-6-methylphenyl)-3-hydroxyphenylmethane, bis (3-cyclohexyl-4-hydroxy-6-methylphenyl)-4-hydroxyphenylmethane, bis(3-cyclohexyl-4-hydroxy-6-methylphenyl)-3,4-dihydroxyphenylmethane, bis (3-cyclohexyl-6-hydroxyphenyl)-3-hydroxyphenylmethane, bis(3-cyclohexyl-6-hydroxyphenyl)-4-hydroxyphenylmethane, bis (3-cyclohexyl-6-hydroxyphenyl)-2-hydroxyphenylmethane, bis (3-cyclohexyl-6-hydroxy-4-methylphenyl)-2-hydroxyphenylmethane, bis (3-cyclohexyl-6-hydroxy-4-methylphenyl)-4-hydroxyphenylmethane, bis (3-cyclohexyl-6-hydroxy-4-methylphenyl)-3,4-dihydroxyphenylmethane, bis(4-hydroxy-2,3,5-trimethylphenyl)-2-hydroxyphenylmethane, bis(4-hydroxy-2,3,5-trimethylphenyl)-3-hydroxyphenylmethane, bis(4-hydroxy-2,3,5-trimethylphenyl)-4-hydroxyphenylmethane, bis(4-hydroxy-2,3,5-trimethylphenyl)-3,4-dihydroxyphenylmethane, bis(4-hydroxy-2,3,5-trimethylphenyl)-4-hydroxy-3-methoxyphenylmethane, and other trisphenolic polyphenolic compounds.
The amount of Ingredient (B) in the invented positive photoresist composition is preferably from 10% to 60% by weight, and more preferably from 20% to 50% by weight relative to the total weight of the alkali-soluble resin ingredient (A), and a sensitizer (intensifier) ingredient (C) added according to necessity as mentioned below. If the amount of Ingredient (B) is less than the above range, images in exact accordance with patterns cannot be obtained, and transferring property is deteriorated. In contrast, if the amount of Ingredient (B) exceeds the above specified range, sensitivity and uniformity of the resulting resist film may be deteriorated, and definition may be decreased.
(C) Sensitizer (Intensifier)
Sensitizers (intensifiers) for use as Ingredients (C) in the invented photoresist composition are not specifically limited, and any known sensitizers such as the polyphenolic compounds of Formula (II) above can be used. Of these sensitizers, preferred compounds are, for example, bis(4-hydroxy-2,3,5-trimethylphenyl)-2-hydroxyphenylmethane, 1,4-bis[1-(3,5-dimethyl-4-hydroxyphenyl)isopropyl]benzene, 2,4-bis(3,5-dimethyl-4-hydroxyphenylmethyl)-6-methylphenol, bis(4-hydroxy-3,5-dimethylphenyl)-2-hydroxyphenylmethane, bis(4-hydroxy-2,5-dimethylphenyl)-2-hydroxyphenylmethane, bis(4-hydroxy-3,5-dimethylphenyl)-3,4-dihydroxyphenylmethane, 1-[1-(4-hydroxyphenyl)isopropyl]-4-[1,1-bis(4-hydroxyphenyl)ethyl]benzene, 1-[1-(3-methyl-4-hydroxyphenyl)isopropyl]-4-[1,1-bis(3-methyl-4-hydroxyphenyl)ethyl]benzene, 2,6-bis[1-(2,4-dihydroxyphenyl)isopropyl]-4-methylphenol, 4,6-bis[1-(4-hydroxyphenyl)isopropyl]resorcin, 4,6-bis(3,5-dimethoxy-4-hydroxyphenylmethyl)pyrogallol, 4,6-bis(3,5-dimethyl-4-hydroxyphenylmethyl)pyrogallol, 2,6-bis(3-methyl-4,6-dihydroxyphenylmethyl)-4-methylphenol, 2,6-bis(2,3,4-trihydroxyphenylmethyl)-4-methylphenol, and 1,1-bis(4-hydroxyphenyl)cyclohexane.
Among them, bis(4-hydroxy-2,3,5-trimethylphenyl)-2-hydroxyphenylmethane, bis(4-hydroxy-3,5-dimethylphenyl)-3,4-dihydroxyphenylmethane, 2,4-bis(3,5-dimethyl-4-hydroxyphenylmethyl)-6-methylphenol and 4,6-bis[1-(4-hydroxyphenyl)isopropyl]resorcin are typically preferred.
Alternately, compounds represented by following Formula (IV) are also preferred: 
wherein x denotes 0 or 1.
When Ingredient (C) is incorporated in the invented positive photoresist composition, the content of Ingredient (C) is preferably 5% to 50% by weight, and more preferably 10% to 35% by weight, relative to the weight of the alkali soluble resin ingredient (A). When the sensitizer (intensifier) (C) is used in the invented positive photoresist composition within the above range, the resulting composition has further improved exposure margin, definition, and DOF properties and exhibits a satisfactory sensitivity.
In addition to the above specified ingredients, where necessary, the invented positive photoresist composition may further comprise any of compatible additives including, for example, ultraviolet absorbents for inhibition of halation and surfactants for prevention of striation within ranges not adversely affecting the objects of the invention. Such ultraviolet absorbents include, for example, 2,2xe2x80x24,4xe2x80x2-tetrahydroxybenzophenone, 4-dimethylamino-2xe2x80x2,4xe2x80x2-dihydroxybenzophenone, 5-amino-3-methyl-1-phenyl-4-(4-hydroxyphenylazo)pyrazole, 4-dimethylamino-4xe2x80x2-hydroxyazobenzene, 4-diethylamino-4xe2x80x2-ethoxyazobenzene, 4-diethylaminoazobenzene and curcumin. The surfactants include, for example, Fluorad FC-430 and FC-431 (trade names, available from Fluorochemical-Sumitomo 3M Co.), F-TOP EF122A, EF122B, EF122C, and EF126 (trade names, available from Tohkem Products Corporation) and other fluorine-containing surfactants.
The invented positive photoresist composition is preferably used as a solution obtained by dissolving each of Ingredients (A) and (B), and where necessary Ingredient (C) and other additional ingredients in an appropriate solvent. Such solvents include those conventionally used in positive photoresist compositions, such as acetone, methyl ethyl ketone, cyclohexanone, methyl isoamyl ketone, 2-heptanone, and other ketones; ethylene glycol, propylene glycol, diethylene glycol, ethylene glycol monoacetate, propylene glycol monoacetate, diethylene glycol monoacetate, or monomethyl ethers, monoethyl ethers, monopropyl ethers, monobutyl ethers or monophenyl ethers thereof, and other polyhydric alcohols and derivatives thereof; dioxane and other cyclic ethers; and ethyl lactate, methyl acetate, ethyl acetate, butyl acetate, methyl pyruvate, ethyl pyruvate, methyl methoxypropionate, ethyl ethoxypropionate, and other esters. Each of these solvents can be used alone or in combination. of these solvents, typically preferred solvents are acetone, methyl ethyl ketone, cyclohexanone, methyl isoamyl ketone, 2-heptanone, and other ketones; and ethyl lactate, methyl acetate, ethyl acetate, butyl acetate, methyl pyruvate, ethyl pyruvate, methyl methoxypropionate, ethyl ethoxypropionate, and other esters.
Practically, the invented positive photoresist composition may be preferably used, for example, in the following manner: Each of Ingredients (A), (B), (C) and other ingredients added according to necessity is dissolved in an appropriate solvent as mentioned above to yield a coating solution; the coating solution is then applied, using a spinner or the like, onto a substrate such as a silicon wafer or a substrate on which an antireflection coating has been formed, and is then dried to form a photosensitive layer; next, the photosensitive layer is irradiated and exposed to light from an ultraviolet source such as a low-pressure mercury lamp, a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, an arc lamp, or a xenon lamp, through a desired mask pattern, or irradiated with a scanning electron beam; and the exposed portions of the film are then dissolved and removed by dipping the substrate in a developer solution, for example, an alkaline aqueous solution such as a 1% to 10% by weight tetramethylammonium hydroxide (TMAH) aqueous solution, thus forming an image being in exact accordance with the mask pattern.